Democratic presidential candidate Hillary Clinton boards her campaign plane at Tampa International Airport in Tampa, Fla., Tuesday. The last stretch of the campaign is looking to be more volatile than usual.

Buckle up. The last two months of this year’s presidential election could well be more volatile than they’ve been in past US political cycles.

Partly that’s due to the personality of one of the contenders, of course. “Donald Trump” is pretty much a synonym for “surprise.” Who knows what the unpredictable GOP nominee will say or do over the next 60-odd days?

But there’s a structural factor that could destabilize polls as well: the undecideds and discontenteds. This year an unusually large percentage of the electorate says it hasn’t made up its mind, or will vote for a third-party candidate. That’s a big chunk of folks who might swing in one way or another when the pressure of choosing on Election Day actually nears.

And Libertarian Gary Johnson and Green Party standard-bearer Jill Stein between them get about 12 percent in four-person national polls, according to the RealClearPolitics rolling average. So, roughly speaking, you’ve got a quarter of the electorate that’s for neither Mr. Trump nor Hillary Clinton.

“We are seeing a historically high number of potential voters who aren’t committing to either major party candidate at this point,” writes Middlebury College political science professor Matthew Dickinson on his “Presidential Power” blog.

Hmmm – perhaps that’s because those major party candidates have historically high unfavorable ratings. Even many supporters aren’t wild about them. A significant share of Trump and Clinton voters say their vote will be based more on which candidate they are against, rather than which they are for, according to Pew Research. That’s much different than the last times two non-incumbents vied for the White House, in 2008 and 2000.

Bottom line: Don’t be surprised if the polls veer (or drift) one way or another. The undecideds should start making up their minds. Some voters will undoubtedly stick with Mr. Johnson and Ms. Stein, but history shows that third party candidates generally lose adherents as campaigns grind to their conclusions.

Will Trump or Mrs. Clinton benefit disproportionately from the undecided overhang? That’s hard to say. There is some evidence that there are more Republicans than Democrats among those who say they haven’t made up their minds or have fled to third parties. Many more of them would prefer the GOP kept control of Congress than that it flips to the Democrats, according to the Wall Street Journal’s data.

That fits with the notion that Trump has split the party and a #NeverTrump faction has refused to fall in line with the nominee. It’s possible that in weeks to come the prospect of President Clinton will drive some of these voters back to Trump. That’s what seems to be happening at the moment. The national polls are tightening in part because Trump is improving his position among self-described Republicans.

But in general, undecided voters tend to be split: about a third lean Republican, a third lean Democrat, and a third are truly torn. They may not vote at all – if you’re not for anyone, why show up at the polls? In the end, they’re an X factor, a known unknown, a plot point in the final act of a drama that concludes in November.